1. Field of the Invention
This invention concerns polymeric films, and in particular polymeric films for the storage or packaging of plant materials.
2. Description of the Prior Art
During storage, plant materials continue to respire even when the materials have been removed from the plant on which they were growing or when the plant material has been dug out of the ground. Thus fruit and vegetables, for example, continue to place demands on the surrounding atmosphere during storage, and deterioration of the quality of the plant materials occurs through water loss and surrounding levels of oxygen and carbon dioxide which do not favour their remaining fresh.
The freshness of fruit and vegetables can be prolonged by packaging, and this can have the added advantage of reducing damage when the fresh produce is displayed on a supermarket shelf. However, there are problems with the use of many packaging materials as the atmosphere within the package changes as respiration proceeds. This can be a particular problem with plant materials which undergo a climacteric stage during ripening, when a sharp rise in the rate of respiration occurs. Thus, while polymeric films, e.g. polyolefin films, can improve the shelf life of fruit and vegetables, a point can come during their storage when deterioration is accelerated by the changes in the atmosphere within the package.
Various proposals have been made for overcoming the problems with storing plant materials in packages made from polymeric films. British Patent Specifications 1106265 and 1134667, for example, describe control of the atmosphere within a package so that the oxygen content is less than that of normal air while the carbon dioxide content is greater than that of normal air, this being effected by the use of imperforate polyethylene sheet of a thickness that it is permeable to oxygen and carbon dioxide and of an area sufficient to allow the sealed-in produce to establish and maintain a controlled atmosphere within the package. Although oxygen and carbon dioxide levels are controlled by this method, the water content of the atmosphere is not and this can lead to undesirable water levels which can increase deterioration of the packaged materials.
Films with very high water permeability are proposed in Japanese Patent Publication 62.148247, 50 to 300 holes per square centimetre being made in the film, each hole being from 50 to 300 microns in diameter. These films are proposed for wrapping cut flowers where the water vapour permeability has to be sufficient to remove condensed water droplets.
Other proposals include the use of as and water-vapour impermeable films which have permeable windows let into them, the windows being made of more permeable materials. Alternatively, composite containers have been proposed in which one side of the container is made from an impervious plastics film and another side is made from a microporous film.